How much memory do you have? And how fast is your hard drive and system bus?
Bink videos are dynamically decompressed and loaded into memory, buffering new data into memory from the hard drive as the movie plays. If however your hard drive or memory is slow, the video will catch up to your buffer resulting in some pauses as your PC struggles to buffer the new data.
Though many people don't realize it, playing video is very system intensive. Consoles fair better because they typically have dedicated video hardware.